We have developed a unique system of in vitro erythropoiesis which provides a source of erythroid peogenitors which (1) are relatively synchronous in their stage of development, (2) proliferate and partially differentiate without erythropoietin (EP) but only differentiate into erythrocytes when they are exposed to EP, and (3) grow in clusters which allows the collection of pure populations of EP sensitive cells in numbers sufficient for biochemical analyses. We have already demonstrated with our system that EP controls multiple events of late differentiation. Our long term objective is to determine how erythropoietin causes differentiation. We propose to further our understanding of the molecular mechanisms of erythropoiesis and EPs action by research in several areas: (1) analysis of how EP causes the accumulation of globin mRNA by measuring transcription and turnover rates of the globin-specific RNA as well as changes in chromatin structure in the vicinity of the globin genes, (2) analysis by two dimensional gel electrophoresis of changes in gene expression as reflected in the proteins synthesized during erythroid development and (3) analysis of the synthesis and fate of heterogeneous nuclear RNA (as detected by monitoring those sequences homologous to the repetitive Alu-related DNA sequences) as the erythroblasts undergo terminal differentiation.